1. Field of the Invention
The present invention relates to coin telephone service for digital network telephone offices and more particularly to a circuit for inserting digital data representative of coin telephone control signals, in the bit stream of a pulse code modulated transmission line connected to a coin telephone.
2. Description of the Prior Art
Coin telephones are controlled by high voltage signals such as ringing current, coin collect and coin return. These signals cannot be transmitted directly over digital subscriber carrier. Heretofore, when coin telephone service is provided over subscriber carrier, a special office channel unit must be provided on the central office end of the transmission line. This special channel unit must accept coin control signals from the switching system and encode them as digital messages over the PCM transmission line.
When coin telephone service is provided over pulse code modulated transmission lines from a digital central office, the office channel unit or channel bank which is required for analog switching systems, is no longer necessary. It can be eliminated if digital data representative of coin telephone control signal are sent directly from the digital central office to a subscriber channel unit at the end of the PCM transmission line.
Most of the pulse code modulated transmission systems use a standardized technique of transmitting 24 channels of 8 bit words over one transmission line. One additional bit, the S-bit (or sync bit), is sent with these 192 information bits to form a 193 bit frame of data transmitted every 125 microseconds. The S-bit is used by the receiving end to determine which bits are assigned to a given channel. It has also been standardized that the least significant bit of the 8 bit word for each channel will be a signalling bit in every sixth frame.
In order to transmit signalling information for a coin telephone over such a line without using analog signals and analog-to-digital converters, digital data representative of coin telephone control signals must be inserted in the least significant bit of each channel word associated with a coin telephone during every sixth frame of 193 bits sent over the pulse code modulated transmission line.
Since this circuit is used in digital central offices transmitting over pulse code modulated transmission lines using digital interface circuits rather than analog-to-digital converters, these lines are designated digital subscriber lines.
Accordingly, it is the object of this invention to provide a digital coin circuit that will control the transmission of digital bits, representative of coin telephone control signals, from the central processing unit to the appropriate position in the bit stream of a pulse code modulated transmission line.